A number of approaches have been devised to measure the thermal conductivity, thermal diffusivity, specific heat and fluid velocity of a fluid or solid of interest. Typically, these and other properties are detected through the use of various types of detectors including resistive bridge type sensors.
One approach for determining thermal conductivity involves the use of a heated element in one leg of a Wheatstone Bridge. The heated element can be placed or positioned in a cavity through which the sample fluid of interest is passed. The heated element is used to introduce a series of amounts of thermal energy into the fluid of interest at various levels by varying the input voltage via alternating current, voltage or power so that the changes in dissipated power caused by changes in the thermal properties of the surrounding fluid or solid (e.g. powder) can be detected as voltage, current or resistance change signals. One of the measurable fluid properties is the thermal conductivity of the fluid.
Further to the measurement of thermally induced changes in electrical resistance, as will be discussed in greater detail below, very small and very accurate “micro bridge” or “micro membrane” semiconductor chips supporting sensors have been implemented in the context of heaters and sensors. Such sensors might include, for example, a pair of thin film sensor elements around a thin film heater element for measuring flow rates. An example of a semiconductor chip sensor configuration is disclosed U.S. Pat. No. 6,361,206, “Microsensor Housing”, which issued to Ulrich Bonne on Mar. 26, 2002, and which is incorporated herein by reference. It can be appreciated that U.S. Pat. No. 6,361,206 is referenced herein for generally illustrative and background purposes only, and should not be considered a limiting feature of the present invention.
Another approach for measuring the thermal conductivity, thermal diffusivity and specific heat of a fluid involves the use of a micro bridge structure that has a heater film and at least one spaced sensor films. A pulse of electrical energy can be applied to the heater at a level and duration such that both a transient change and a substantially steady-state temperature occur at the sensor.
The thermal conductivity of the fluid of interest is determined based upon a known relation between the sensor output and the thermal conductivity at steady-state sensor temperatures. The specific heat and thermal diffusivity of the fluid of interest are determined based on a known relation among the thermal conductivity, the rate of change of the sensor output during a transient temperature change in the sensor, and the thermal diffusivity and specific heat.
A typical approach for determining the velocity of a fluid of interest is to determine the time require for a thermal wave to flow from a source heater element to a destination sensor element. By knowing the distance between the heater element and the sensor element, the velocity of the fluid can be calculated.
The thermal waves can propagate through the fluid at a rate that is dependent on the fluid velocity flowing perpendicular to the heater strip. A thermo-electric detector, spaced from one or both side of the heater, senses the thermal wave and provides a corresponding detector output signal. The velocity of the fluid is determined, at least to first order, from the time differential between the heater input signal and the detector output signal.
In many instances it may be necessary to achieve temperature compensation for fluid properties, such as thermal conductivity, viscosity, pressure, vapor concentration, and so forth. Heretofore, temperature compensation has involved digitally processing independent signals of property and temperature, preferably taken at two temperatures, and requiring a fair amount of time. Other methods, such as an orientation sensor and many of the aforementioned devices, provide no compensation at all. The ability to achieve temperature compensation properly for fluid sensors, such as the fluid sensors discussed above, and other similar sensing devices, is an important objective that has not yet been achieved. The methods and systems disclosed herein provide a solution to the lack of temperature compensation associated with current and traditional thermal property and other fluid property sensors.